The use of solenoids to actuate mechanical devices, such as valves, upon application of electrical energy is well known. Conventional solenoids, wherein a plunger is moved from a deenergized position to an energized position, require continuous application of an electric current to the solenoid winding to maintain the plunger in the latter position. Furthermore, in order to return the plunger to its deenergized position upon termination of the electrical current, means for generating a restoring force such as springs must be employed. The use of such springs is disadvantageous in that they further increase the energy requirements of the solenoid requiring that sufficient electrical current be applied to the actuating coil to overcome the counterforces of the spring. Springs are also subject to corrosion and wear and subsequent mechanical failure.
To overcome these disadvantages, devices containing a magnetic latching feature have been developed. In these devices the plunger is held in position by the force of a permanent magnet and may be shuttled from one position to another by conventional mechanical forces or by application of electrical energy to an electromagnetic winding to create a magnetic force which counteracts and overcomes the permanent magnetic force holding the plunger in a given position. In such devices, the construction is often such that the magnetically responsive plunger is not visible to an observer of the apparatus in which it is employed.
In apparatus where a mechanical member, hidden from view, is to move from one position to another it is often desirable to verify such movement by some remote sensing means. The use of a permanent magnet and a reed switch, one mounted on the moving member and the other stationary is well known. In such configurations the magnet and reed switch are moved relative to one another, the reed switch closing when adjacent to the permanent magnet and opening when separated from it.
Instead of using permanent magnets to indicate a shift in position of a moving member, the prior art also teaches the use of an iron member, affixed to the moving member, situated within the region of the energizing coil so that the iron member is magnetized by the energized coil thereby serving a function similar to that of the permanent magnet when moved relative to an adjacent reed switch. This configuration while sometimes useful for position indication has several inherent disadvantages. First, the sensing reed switch must be adjacent to the moving magnetic member when the member is in one position and displaced from that member when it is moved to its alternate position. This places substantial structural limitations upon the solenoid mechanism. Furthermore, in order to verify movement of the plunger to a desired position, in a solenoid having a duty cycle during which the plunger alternates between positions, the indicator operated by the reed switch sensor must be observed at a time when the moving member is to be in the position to be sensed. The reed switch will not be actuated at all if the solenoid plunger fails to move as required, and unless the operator is aware that the solenoid winding effecting required plunger movement was energized, there is no way of determining system failure.